how does one cancel the automatic assignment of planes to the air missions and get to select them manually? It's probably quite obviuous but I've been struggling with that for a while, can find anything in the manual

Well, if you don't want them to fly any mission, select stand-down or training. For manual use, select the mission AND the target you want them to fly. If you DON'T select a target, the commander of the air-group will order his fly-boys to attack what he seems to be fit.

Hold down the shift key and select target for the mission, but thats working only for bombing, supply missions etc. If you want assign certain groups to ground support, as far as I know thats not possible.